Association of activated phosphatidylinositol 3-kinase with p120cbl in antigen receptor-ligated B cells

Cbl ubiquitination of p85 is essential for Epo-induced EpoR endocytosis

Erythropoietin (Epo) binding to the Epo receptor (EpoR) elicits downstream signaling that is essential for red blood cell production. One important negative regulatory mechanism to terminate Epo signaling is Epo-induced EpoR endocytosis and degradation. Defects in this mechanism play a key role in the overproduction of erythrocytes in primary familial and congenital polycythemia (PFCP). Here we have identified a novel mechanism mediating Epo-dependent EpoR internalization. Epo induces Cbl-dependent ubiquitination of the p85 regulatory subunit of PI3K, which binds to phosphotyrosines on EpoR. Ubiquitination allows p85 to interact with the endocytic protein epsin-1, thereby driving EpoR endocytosis. Knockdown of Cbl, expression of its dominant negative forms, or expression of an epsin-1 mutant devoid of ubiquitin-interacting motifs all compromise Epo-induced EpoR internalization. Mutated EpoRs mimicking those from PFCP patients cannot bind p85, co-localize with epsin-1, or internalize on Epo stimulation and exhibit Epo hypersensitivity. Similarly, knockdown of Cbl also causes Epo hypersensitivity in primary erythroid progenitors. Restoring p85 binding to PFCP receptors rescues Epo-induced epsin-1 co-localization and EpoR internalization and normalizes Epo hypersensitivity. Our results uncover a novel Cbl/p85/epsin-1 pathway in EpoR endocytosis and show that defects in this pathway contribute to excessive Epo signaling and erythroid hyperproliferation in PFCP.

Beta2-integrin-mediated adhesion and intracellular Ca2+ release in human eosinophils

Human eosinophils spontaneously adhere to various substrates in the absence of exogenously added activators. In the present study a method was developed for characterizing eosinophil adhesion by measuring changes in impedance. Impedance measurements were performed in HCO(3)-buffered HybriCare medium maintained in a humidified 5% CO(2) incubator at 37 degrees C. Impedance increased by more than 1 kOmega within minutes after eosinophils made contact with the substrate, reaching a peak within 20 min. Blocking mobilization of intracellular [Ca(2+)] that precedes adhesion with BAPTA-AM (10 microM) completely inhibited the rise in impedance as well as the changes in cell shape typically observed in adherent cells. However, lowering the extracellular [Ca(2+)] with 2.5 mM EGTA did not inhibit the increase in impedance. Pretreatment with anti-CD18 antibody to block substrate interactions with beta(2)-integrins, or jasplakinolide (2 microM) to block actin reorganization, abolished the increase in impedance and adherent morphology of the cells. Exposure of eosinophils to the phosphatidylinositol 3 kinase inhibitor LY294002 (5 microM) or treatment with protein kinase C zeta pseudosubstrate to competitively inhibit activity of the enzyme significantly reduced the increase in impedance and inhibited the cell spreading associated with adhesion. These results demonstrate a novel method for measuring eosinophil adhesion and showed that, following formation of a tethered attachment, a rapid increase in intracellular [Ca(2+)] precedes the cytoskeletal rearrangements required for cell shape changes and plasma membrane-substrate interactions associated with adhesion.

Ligand-independent signaling during early avian B cell development

Surface immunoglobulin (sIg) expression has been conserved as a critical checkpoint in B lymphocyte development. In the chicken embryo, only sIg+ B cells are selectively expanded in the bursa of Fabricius, a primary lymphoid organ unique to the avian species. We have previously demonstrated that an interaction between the antigen- binding sites of sIg and a specific bursal ligand(s) is not required to regulate this developmental checkpoint. Rather, the requirement for sIg expression can be attributed to the surface expression of the Igalpha/beta heterodimer associated with sIg. More specifically, ligand-independent signaling downstream of the Igalpha cytoplasmic domain drives all bursal stages of B cell development during embryogenesis. We discuss here a site-directed mutagenesis approach to identify the critical membrane proximal events involved in ligand-independent signaling during B cell development.

The Cbl Family and Other Ubiquitin Ligases

Regulation of tyrosine kinase-mediated cellular activation through antigen receptors is of great biological and practical significance. The evolutionarily conserved Cbl family ubiquitin ligases have emerged as key negative regulators of activated tyrosine kinase-coupled receptors, and their impaired function switches a normal immune response into autoimmunity. Cbl proteins facilitate the ubiquitinylation of activated tyrosine kinases and other signaling proteins and of the signaling chains of receptors themselves; monoubiquitin tag promotes sorting of activated receptors and associated proteins into internal vesicles of the multivesicular body, facilitating their lysosomal degradation, whereas polyubiquitin tag promotes proteasomal degradation. Notably, increased expression of Cbl proteins and other ubiquitin ligases is a component of anergic signaling program in T cells. Thus, controlled destruction of the signaling apparatus has emerged as a key to fine-tuning antigen receptor signaling. Further studies of this pathway are likely to elucidate the pathogenesis of autoimmune diseases and offer new therapeutic targets.

c-Cbl Is Involved in Met Signaling in B Cells and Mediates Hepatocyte Growth Factor-Induced Receptor Ubiquitination

Hepatocyte growth factor/scatter factor (HGF) and its receptor tyrosine kinase Met are key regulators of epithelial motility and morphogenesis. Recent studies indicate that the HGF/Met pathway also plays a role in B cell differentiation, whereas uncontrolled Met signaling may lead to B cell neoplasia. These observations prompted us to explore HGF/Met signaling in B cells. In this study, we demonstrate that HGF induces strong tyrosine phosphorylation of the proto-oncogene product c-Cbl in B cells and increases Cbl association with the Src family tyrosine kinases Fyn and Lyn, as well as with phosphatidylinositol-3 kinase and CrkL. In addition, we demonstrate that c-Cbl mediates HGF-induced ubiquitination of Met. This requires the juxtamembrane tyrosine Y1001 (Y2) of Met, but not the multifunctional docking site (Y14/15) or any additional C-terminal tyrosine residues (Y13-16). In contrast to wild-type c-Cbl, the transforming mutants v-Cbl and 70Z/3 Cbl, which lack the ubiquitin ligase RING finger domain, suppress Met ubiquitination. Our findings identify c-Cbl as a negative regulator of HGF/Met signaling in B cells, mediating ubiquitination and, consequently, proteosomal degradation of Met, and suggest a role for Cbl in Met-mediated tumorigenesis.

Defective proliferative responses in B lymphocytes and thymocytes that lack neurofibromin

Nf1(-/-) fetal liver cells were used to reconstitute B and T cells in Rag-1(-/-) mice. Lymphocyte development was largely unimpaired in the absence of neurofibromin. However antigen-receptor induced proliferation was defective in neurofibromin deficient peripheral B cells and CD4(+) single positive thymocytes. In contrast to its role as a negative regulator of proliferation in many other cell types, neurofibromin may be a positive regulator of lymphocyte proliferation. Peripheral B cells exhibited circumscribed defects in anti-IgM induced protein tyrosine phosphorylation, which may contribute to the unexpected proliferative defect seen in these cells.

Beyond the RING: CBL proteins as multivalent adapters

Following discovery of c-Cbl, a cellular form of the transforming retroviral protein v-Cbl, multiple Cbl-related proteins have been identified in vertebrate and invertebrate organisms. c-Cbl and its homologues are capable of interacting with numerous proteins involved in cell signaling, including various molecular adapters and protein tyrosine kinases. It appears that Cbl proteins play several functional roles, acting both as multivalent adapters and inhibitors of various protein tyrosine kinases. The latter function is linked, to a substantial extent, to the E3 ubiquitin-ligase activity of Cbl proteins. Experimental evidence for these functions, interrelations between them, and their biological significance are addressed in this review, with the main accent placed on the adapter functions of Cbl proteins.

The Gab1 docking protein links the b cell antigen receptor to the phosphatidylinositol 3-kinase/Akt signaling pathway and to the SHP2 tyrosine phosphatase

B cell antigen receptor (BCR) signaling causes tyrosine phosphorylation of the Gab1 docking protein. This allows phosphatidylinositol 3-kinase (PI3K) and the SHP2 tyrosine phosphatase to bind to Gab1. In this report, we tested the hypothesis that Gab1 acts as an amplifier of PI3K- and SHP2-dependent signaling in B lymphocytes. By overexpressing Gab1 in the WEHI-231 B cell line, we found that Gab1 can potentiate BCR-induced phosphorylation of Akt, a PI3K-dependent response. Gab1 expression also increased BCR-induced tyrosine phosphorylation of SHP2 as well as the binding of Grb2 to SHP2. We show that the pleckstrin homology (PH) domain of Gab1 is required for BCR-induced phosphorylation of Gab1 and for Gab1 participation in BCR signaling. Moreover, using confocal microscopy, we show that BCR ligation can induce the translocation of Gab1 from the cytosol to the plasma membrane and that this requires the Gab1 PH domain as well as PI3K activity. These findings are consistent with a model in which the binding of the Gab1 PH domain to PI3K-derived lipids brings Gab1 to the plasma membrane, where it can be tyrosine-phosphorylated and then act as an amplifier of BCR signaling.

The adapter type protein CMS/CD2AP binds to the proto-oncogenic protein c-Cbl through a tyrosine phosphorylation-regulated Src homology 3 domain interaction

CMS/CD2AP is a cytoplasmic protein critical for the integrity of the kidney glomerular filtration and the T cell function. CMS contains domains and motifs characteristic for protein-protein interactions, and it is involved in the regulation of the actin cytoskeleton. We report here that the individual SH3 domains of CMS bind to phosphotyrosine proteins of approximately 80, 90, and 180 kDa in cell lysates stimulated with epidermal growth factor. The second SH3 domain of CMS bound specifically to a tyrosine-phosphorylated protein of 120 kDa, which we identified as the proto-oncoprotein c-Cbl. The c-Cbl-binding site for CMS mapped to the carboxyl terminus of c-Cbl and is different from the proline-rich region known to bind SH3-containing proteins. CMS binding to c-Cbl was markedly attenuated in a tyrosine phosphorylation-defective c-Cbl mutant indicating that this interaction is dependent on the tyrosine phosphorylation of CMS. It also implies that CMS interacts with c-Cbl in an inducible fashion upon stimulation of a variety of cell-surface receptors. Immunofluorescence analysis revealed that both proteins colocalize at lamellipodia and leading edges of cells, and we propose that the interaction of CMS with c-Cbl offers a mechanism by which c-Cbl associates and regulates the actin cytoskeleton.

Differential interaction of Cbl with Grb2 and CrkL in CD2-mediated NK cell activation

Natural killer (NK) cells participate in both innate and adoptive immunity by their prompt secretion of cytokines and by their ability to lyse virally infected cells or tumor cells. CD2 is surface glycoprotein receptors and crucial for NK cell activation. However, molecular events involved in CD2-mediated NK cell activation have not been fully elucidated. Cbl-Grb2 and Cbl-CrkL interactions have been implicated in T cell and B cell receptor, and cytokine receptor signaling. Here we analyzed tyrosine phosphorylation and interactions of Cbl with adapter proteins, Grb2 and CrkL, in NK3.3 cells. CD2 crosslinking results in the marked tyrosine phosphorylation of Cbl in an antibody concentration- and time-dependent manner. Immunodepletion studies reveal that Grb2-associated tyrosine phosphorylated p120 kDa protein is Cbl. In vitro binding studies using GST-fusion proteins demonstrate that Cbl constitutively associates with the SH3 domains of Grb2, with a preference for the amino-terminal domain. In addition, we demonstrate that CrkL associates with a large portion of tyrosine phosphorylated Cbl after CD2 stimulation of NK3.3 cells. In contrast to constitutive Cbl association with Grb2, tyrosine phosphorylated Cbl interacts with CrkL via its SH2 domain only after CD2 stimulation. Although the precise roles of interactions of Cbl with Grb2 and CrkL in NK cell activation remains to be elucidated, their tyrosine phosphorylation, in addition to the multiple protein interactions described here, strongly suggest that interactions of Cbl with Grb2 and CrkL may play pivotal roles in CD2-mediated NK cell activation.

Adapter proteins in lymphocyte antigen-receptor signaling

Adapter molecules contain discrete modular domains that direct specific intermolecular interactions to orchestrate assembly of signaling complexes. A number of adapter proteins play critical roles in both positive and negative regulation of antigen-receptor signaling, influencing lymphocyte development and function.

Role of phosphatidylinositol 3-kinase and specific protein kinase B isoforms in the suppression of apoptosis mediated by the Abelson protein-tyrosine kinase

Leukemogenic oncogenes, such as the Abelson protein-tyrosine kinases (PTK), disrupt the normal regulation of survival, proliferation, and differentiation in hemopoietic progenitor cells. In the absence of cytokines, hemopoietic progenitor cells die by apoptosis. Abl PTKs mediate suppression of this apoptotic response leading to aberrant survival. To investigate the mechanism of Abl PTK action, we have used an interleukin-3-dependent murine mast cell line that expresses a temperature-sensitive form of the v-ABL PTK, which is active at the permissive temperature of 32 degrees C and inactive at 39 degrees C. At the permissive temperature, these cells are resistant to apoptosis induced both by the withdrawal of the hemopoietic growth factor (interleukin-3) and the addition of cytotoxic drugs. We demonstrate that v-Abl associates with and stimulates activation of phosphatidylinositol 3-kinase (PI3K) and, crucially, that this activation results in enhanced cellular levels of the mass of the second messenger phosphatidylinositol-3,4,5-trisphosphate. Activation of PI3K leads to enhanced activity of PKB and increased levels of the anti-apoptotic protein Bcl-X(L). Transfection of cells with a dominant negative PKB reduces both the Abl-stimulated PKB activity and the survival effect conferred by activation of this oncogene. Thus, PI3K and PKB are required for the anti-apoptotic effects of Abl PTK.

Differential effects of Cbl and 70Z/3 Cbl on T cell receptor-induced phospholipase Cgamma-1 activity

We demonstrate that the differential effects Cbl and oncogenic 70Z/3 Cbl have on Ca(2+)/Ras-sensitive NF-AT reporters is partially due to their opposing ability to regulate phospholipase Cgamma1 (PLCgamma1) activation as demonstrated by analysis of the activation of an NF-AT reporter construct and PLCgamma1-mediated inositol phospholipid (PI) hydrolysis. Cbl over-expression resulted in reduced T cell receptor-induced PI hydrolysis, in the absence of any effect on PLCgamma1 tyrosine phosphorylation. In contrast, expression of 70Z/3 Cbl led to an increase in basal and OKT3-induced PLCgamma1 phosphorylation and PI hydrolysis. These data indicate that Cbl and 70Z/3 Cbl differentially regulate PLCgamma1 phosphorylation and activation. The implications of these data on the mechanism of Cbl-mediated signaling regulation are discussed.

Cloning and characterization of a novel adaptor protein, CIN85, that interacts with c-Cbl

The c-Cbl protooncogene product is a prominent substrate of protein tyrosine kinases and is rapidly tyrosine-phosphorylated upon stimulation of a wide variety of cell-surface receptors. We have identified a novel c-Cbl-interacting protein termed CIN85 with a molecular mass of 85 kDa which shows similarity to adaptor proteins, CMS and CD2AP. CIN85 mRNA is expressed ubiquitously in normal human tissues and cancer cell lines analyzed. CIN85 was basally associated with c-Cbl. For interaction of CIN85 with c-Cbl, the second SH3 domain of CIN85 was shown to serve as a central player. The CIN85-c-Cbl association was enhanced shortly after stimulation of 293 cells with epidermal growth factor (EGF) and gradually diminished to a basal level, which correlated with a tyrosine phosphorylation level of c-Cbl. Our results suggest that CIN85 may play a specific role in the EGF receptor-mediated signaling cascade via its interaction with c-Cbl.

SYK is upstream of phosphoinositide 3-kinase in B cell receptor signaling

We have recently demonstrated that the D3-phosphoinositide phosphatidylinositol 3,4,5-trisphosphate (PtdIns-3,4,5-P(3)) is critical for producing sustained calcium signals through its role in promoting the function of TEC family tyrosine kinases such as Bruton's tyrosine kinase. Although PtdIns-3,4,5-P(3) can potentially be synthesized by any of several types of phosphoinositide 3-kinases (PI3Ks), B cell receptor (BCR)-induced PtdIns-3,4,5-P(3) production is thought to occur primarily through the activation of the class Ia (p85/p110) PI3Ks. This process has been proposed to be mediated by an interaction between the Src family kinase LYN and the p85 subunit of PI3K and/or through p85 membrane recruitment mediated by CBL and/or CD19. However, calcium signaling and other PI3K-dependent signals are relatively preserved in a LYN kinase-deficient B lymphocyte cell line, suggesting that an alternative pathway for PI3K activation exists. As SYK/ZAP70 kinases are upstream from many BCR-initiated signaling events, we directly analyzed SYK-dependent accumulation of both PtdIns-3,4,5-P(3) and PtdIns-3,4-P(2) in B cell receptor signaling using both dominant negative and genetic knockout approaches. Both methods indicate that SYK is upstream of, and necessary for, a significant portion of BCR-induced PtdIns-3,4, 5-P(3) production. Whereas CD19 does not appear to be involved in this SYK-dependent pathway, the SYK substrate CBL is likely involved as the dominant negative SYK markedly attenuates CBL tyrosine phosphorylation and completely blocks the BCR-dependent association of CBL with p85 PI3K.

Role of Src in the Modulation of Multiple Adaptor Proteins in FcRI Oxidant Signaling

Cross-linking of Fc receptors for IgA, FcR (CD89), on monocytes/macrophages is known to enhance phagocytic activity and generation of oxygen free radicals. We provide evidence here that the FcR signals through the γ subunit of FcɛRI in U937 cells differentiated with interferon γ (IFNγ). Our results provide the first evidence that FcR-mediated signals modulate a multimolecular adaptor protein complex containing Grb2, Shc, SHIP, CrkL, Cbl, and SLP-76. Cross-linking of FcRI using anti-FcRI induces the phosphorylation of the γ subunit as detected by mobility retardation on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Stimulation of FcRI induced the tyrosine phosphorylation of Shc and increased the association of Grb2 with Shc and CrkL. Grb2 associates constitutively with Sos, and the latter undergoes mobility shift upon FcRI stimulation. The complex adapter proteins, Cbl and SLP-76, are physically associated in myeloid cells and both proteins undergo tyrosine phosphorylation upon FcR stimulation. These data indicate that the stimulation of FcR results in the modulation of adaptor complexes containing tyrosine-phosphorylated Cbl, Shc, SHIP, Grb2, and Crkl. Experiments performed with the Src kinase inhibitor, PP1, provide the first evidence that Src kinase activation is required for FcRI-induced production of superoxide anions and provide insight into the mechanism for FcR-mediated activation of downstream oxidant signaling in myeloid cells.

Role of Src in the Modulation of Multiple Adaptor Proteins in FcRI Oxidant Signaling

Cross-linking of Fc receptors for IgA, FcR (CD89), on monocytes/macrophages is known to enhance phagocytic activity and generation of oxygen free radicals. We provide evidence here that the FcR signals through the γ subunit of FcɛRI in U937 cells differentiated with interferon γ (IFNγ). Our results provide the first evidence that FcR-mediated signals modulate a multimolecular adaptor protein complex containing Grb2, Shc, SHIP, CrkL, Cbl, and SLP-76. Cross-linking of FcRI using anti-FcRI induces the phosphorylation of the γ subunit as detected by mobility retardation on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Stimulation of FcRI induced the tyrosine phosphorylation of Shc and increased the association of Grb2 with Shc and CrkL. Grb2 associates constitutively with Sos, and the latter undergoes mobility shift upon FcRI stimulation. The complex adapter proteins, Cbl and SLP-76, are physically associated in myeloid cells and both proteins undergo tyrosine phosphorylation upon FcR stimulation. These data indicate that the stimulation of FcR results in the modulation of adaptor complexes containing tyrosine-phosphorylated Cbl, Shc, SHIP, Grb2, and Crkl. Experiments performed with the Src kinase inhibitor, PP1, provide the first evidence that Src kinase activation is required for FcRI-induced production of superoxide anions and provide insight into the mechanism for FcR-mediated activation of downstream oxidant signaling in myeloid cells.

The B Cell Antigen Receptor Activates the Akt (Protein Kinase B)/Glycogen Synthase Kinase-3 Signaling Pathway Via Phosphatidylinositol 3-Kinase

We have previously shown that the B cell Ag receptor (BCR) activates phosphatidylinositol (PI) 3-kinase. We now show that a serine/threonine kinase called Akt or protein kinase B is a downstream target of PI 3-kinase in B cells. Akt has been shown to promote cell survival as well as the transcription and translation of proteins involved in cell cycle progression. Using an Ab that specifically recognizes the activated form of Akt that is phosphorylated on serine 473, we show that BCR engagement activates Akt in a PI 3-kinase-dependent manner. These results were confirmed using in vitro kinase assays. Moreover, BCR ligation also induced phosphorylation of Akt of threonine 308, another modification that is required for activation of Akt. In the DT40 chicken B cell line, phosphorylation of Akt on serine 473 was completely dependent on the Lyn tyrosine kinase, while the Syk tyrosine kinase was required for sustained phosphorylation of Akt. Complementary experiments in BCR-expressing AtT20 endocrine cells confirmed that Src kinases are sufficient for BCR-induced Akt phosphorylation, but that Syk is required for sustained phosphorylation of Akt on both serine 473 and threonine 308. In insulin-responsive cells, Akt phosphorylates and inactivates the serine/threonine kinase glycogen synthase kinase-3 (GSK-3). Inactivation of GSK-3 may promote nuclear accumulation of several transcription factors, including NF-ATc. We found that BCR engagement induced GSK-3 phosphorylation and decreased GSK-3 enzyme activity. Thus, BCR ligation initiates a PI 3-kinase/Akt/GSK-3 signaling pathway.

Tyrosine phosphorylation of C-Cbl facilitates adhesion and spreading while suppressing anchorage-independent growth of V-Abl-transformed NIH3T3 fibroblasts

The protooncogenic protein c-Cbl becomes tyrosine phosphorylated in normal cells in response to a variety of external stimuli, as well as in cells transformed by oncogenic protein tyrosine kinases. Tyrosine phosphorylation of c-Cbl upregulates its binding to multiple crucial signaling molecules. However, the biological consequences of c-Cbl-mediated signaling are insufficiently understood. To analyse the biological functions of c-Cbl, we overexpressed wild-type c-Cbl and its tyrosine phosphorylation-defective mutant form in v-Abl-transformed NIH3T3 fibroblasts. In this system, wild-type c-Cbl facilitated adhesion and spreading of v-Abl-transformed fibroblasts on the extracellular matrix, while reducing anchorage independence of these cells, as measured by their colony-forming efficiency in soft agar. Therefore, overexpression of wild-type c-Cbl exhibits an overall transformation-suppressing effect. By contrast, overexpression of a tyrosine phosphorylation-defective form of c-Cbl increases neither adhesion nor anchorage dependence of v-Abl-transformed fibroblasts. Analysis of the role of individual tyrosine phosphorylation sites of c-Cbl in these phenomena indicates that both phosphatidylinositol-3' kinase and the CrkL adaptor protein may be involved in the observed effects of c-Cbl. To summarize, the results presented in this report indicate that c-Cbl is involved in regulation of cell adhesion and cytoskeletal rearrangements, and that these effects of c-Cbl are dependent on its tyrosine phosphorylation.

Protein Kinase C Activation Inhibits Tyrosine Phosphorylation of Cbl and Its Recruitment of Src Homology 2 Domain-Containing Proteins

One of the major proteins that is rapidly tyrosine phosphorylated upon stimulation of the TCR/CD3 complex is the 120-kDa product of the c-cbl protooncogene (Cbl). Upon activation, tyrosine-phosphorylated Cbl interacts with the Src homology 2 (SH2) domains of several signaling proteins, e.g., phosphatidylinositol 3-kinase (PI3-K) and CrkL. In the present study, we report that pretreatment of Jurkat T cells with PMA reduced the anti-CD3-induced tyrosine phosphorylation of Cbl and, consequently, its activation-dependent association with PI3-K and CrkL. A specific protein kinase C (PKC) inhibitor (GF-109203X) reversed the effect of PMA on tyrosine phosphorylation of Cbl and restored the activation-dependent association of Cbl with PI3-K and CrkL. We also provide evidence that PKCα and PKCθ can physically associate with Cbl and are able to phosphorylate it in vitro and in vivo. Furthermore, a serine-rich motif at the C terminus of Cbl, which is critical for PMA-induced 14-3-3 binding, is also phosphorylated by PKCα and PKCθ in vitro. These results suggest that, by regulating tyrosine and serine phosphorylation of Cbl, PKC is able to control the association of Cbl with signaling intermediates, such as SH2 domain-containing proteins and 14-3-3 proteins, which may consequently result in the modulation of its function.

Perturbed Regulation of ZAP-70 and Sustained Tyrosine Phosphorylation of LAT and SLP-76 in c-Cbl-Deficient Thymocytes

Recent studies indicate that c-Cbl and its oncogenic variants can modulate the activity of protein tyrosine kinases. This finding is supported by studies showing that c-Cbl interacts directly with a negative regulatory tyrosine in ZAP-70, and that the levels of tyrosine-phosphorylated ZAP-70 and numerous other proteins are increased in TCR-stimulated thymocytes from c-Cbl-deficient mice. Here, we demonstrate that this enhanced phosphorylation of ZAP-70 and that of two substrates, LAT and SLP-76, is not due to altered protein levels but is the consequence of two separate events. First, we find increased expression of tyrosine-phosphorylated TCRζ chain in c-Cbl-deficient thymocytes, which results in a higher level of ζ-chain-associated ZAP-70 that is initially accessible for activation. Thus, more ZAP-70 is activated and more of its substrates (LAT and SLP-76) become tyrosine-phosphorylated after TCR stimulation. However, an additional mechanism of ZAP-70 regulation is evident at a later time poststimulation. At this time, ZAP-70 from both normal and c-Cbl−/− thymocytes becomes hyperphosphorylated; however, only in normal thymocytes does this correlate with ZAP-70 down-regulation and a diminished ability to phosphorylate LAT and SLP-76. In contrast, c-Cbl-deficient thymocytes display altered phosphorylation kinetics, for which LAT phosphorylation is increased and SLP-76 phosphorylation is sustained. Thus, the ability to down-regulate the phosphorylation of two ZAP-70 substrates is impaired in c-Cbl−/− thymocytes. These findings provide evidence that c-Cbl is involved in the negative regulation of the phosphorylation of LAT and SLP-76 by ZAP-70.

Dual regulation of T cell receptor-mediated signaling by oncogenic Cbl mutant 70Z

We previously showed that an oncogenic Cbl mutant (70Z) is constitutively active in transcriptional activation of nuclear factor at activated T cells (NFAT). However, the mechanism underlying this effect remains unclear. Here we analyzed the effects of 70Z mutations at an amino-terminal loss of function site (Gly-306) and at carboxyl-terminal potential tyrosine or serine phosphorylation sites on association with signaling proteins and on NFAT activation. Mutation at Gly-306 of 70Z disrupted its association with Zap-70 and almost completely abolished its ability to induce NFAT activation under basal and ionomycin-stimulated conditions. However, mutations at potential tyrosine or serine phosphorylation sites had little effect. In fact, expression of 70Z with Tyr-700, Tyr-731, or Tyr-774 mutated to Phe increased NFAT activity in comparison with unmutated 70Z. These findings suggest that an amino terminus-mediated interaction of 70Z with Zap-70 plays a positive role and that a carboxyl terminus-mediated, phosphotyrosine-dependent interaction with their binding proteins plays a negative role in 70Z-mediated NFAT activation. In support of this notion are the observations that 70Z reduced T cell receptor-induced NFAT activation and that wild-type Cbl further inhibited this event, suggesting that both 70Z and wild-type Cbl employ a similar mechanism by which Cbl proteins dually regulate T cell receptor-mediated signaling.

Fc receptor-mediated platelet activation is dependent on phosphatidylinositol 3-kinase activation and involves p120(Cbl)

The platelet receptor for the Fc domain of IgGs (FcgammaRIIa) triggers intracellular signaling through protein tyrosine phosphorylations leading to platelet aggregation. In this study, we focused on the adaptor protein p120(cbl) (Cbl), which became tyrosine-phosphorylated after platelet activation induced by antibodies. Cbl phosphorylation was dependent on Fc receptor engagement. An association of Cbl with the p85 subunit of phosphatidylinositol 3-kinase (PI 3-K) occurred in parallel with Cbl tyrosine phosphorylation. We showed by in vitro experiments that Cbl/p85 association was mediated by the Src homology 3 domain of p85/PI 3-K and the proline-rich region of Cbl. Inhibition of PI 3-K activity by wortmannin led to the blockade of both platelet aggregation and serotonin release mediated by FcgammaRIIa engagement, whereas it only partly inhibited those induced by thrombin. Thus, PI 3-K may play a crucial role in the initiation of platelet responses after FcgammaRIIa engagement. Our results suggest that Cbl is involved in platelet signal transduction by the recruitment of PI 3-K to the FcgammaRIIa pathway, possibly by increasing PI 3-K activity.

The Gab1 protein is a docking site for multiple proteins involved in signaling by the B cell antigen receptor

Gab1 is a member of the docking/scaffolding protein family which includes IRS-1, IRS-2, c-Cbl, p130(cas), and p62(dok). These proteins contain a variety of protein-protein interaction motifs including multiple tyrosine residues that when phosphorylated can act as binding sites for Src homology 2 (SH2) domain-containing signaling proteins. We show in the RAMOS human B cell line that Gab1 is tyrosine-phosphorylated in response to B cell antigen receptor (BCR) engagement. Moreover, tyrosine phosphorylation of Gab1 correlated with the binding of several SH2-containing signaling proteins to Gab1 including Shc, Grb2, phosphatidylinositol 3-kinase, and the SHP-2 tyrosine phosphatase. Far Western analysis showed that the SH2 domains of Shc, SHP-2, and the p85 subunit of phosphatidylinositol 3-kinase could bind directly to tyrosine-phosphorylated Gab1 isolated from activated RAMOS cells. In contrast, the Grb2 SH2 domain did not bind directly to Gab1 but instead to the Shc and SHP-2 associated with Gab1. We also show that Gab1 is present in the membrane-enriched particulate fraction of RAMOS cells and that Gab1/signaling protein complexes are found in this fraction after BCR engagement. Thus, tyrosine-phosphorylated Gab1 may recruit cytosolic signaling proteins to cellular membranes where they can act on membrane-bound targets. This may be a critical step in the activation of multiple BCR signaling pathways.

c-Cbl: a regulator of T cell receptor-mediated signalling

The 120-kDa protein product of the c-Cbl proto-oncogene is a ubiquitously expressed cytoplasmic protein that is especially abundant in the thymus, indicating an important role for Cbl in thymic signalling. c-Cbl possesses a highly conserved N-terminal phosphotyrosine binding domain, a C3HC4 RING finger motif, multiple proline-rich motifs, and a number of potential tyrosine phosphorylation sites. Cbl is an early and prominent substrate of protein tyrosine kinases following stimulation of a variety of cell surface receptors, and forms constitutive and inducible associations with a wide range of signalling intermediates. Genetic studies of the Cbl homologue Sli-1 in Caenorhabitis elegans predicted a role for Cbl as a negative regulator of protein tyrosine kinase-mediated signalling pathways. Numerous studies have now shown that expression of Cbl and its oncogenic variants can indeed modulate signalling from activated protein tyrosine kinases. The present review highlights some of the recent developments in our understanding of Cbl function, with particular reference to its participation and possible roles in TCR-mediated signalling.

Decreased protein levels of the c-Cbl protooncogene in murine AIDS

Murine acquired immunodeficiency syndrome (MAIDS) can be viewed as a lymphoproliferative disease which involves B cells as well as T cells from spleen and lymph nodes while thymus and Peyer's patches do not participate in the process. The 120-kDa protooncogene product c-Cbl was initially cloned from the murine Cas NS-1 B cell lymphoma. It is a main target of immunoreceptor (TCR and BCR)-mediated protein tyrosine kinase activity. Moreover, recent data suggest that c-Cbl might play a crucial role in the regulation of cell proliferation through regulation of GTP-binding proteins. Therefore, the involvement of c-Cbl was evaluated in the lymphoproliferative disease induced by the MAIDS virus. The expression of the c-Cbl protein was dramatically reduced in the lymph node of infected mice while it remained normal in the thymus. In contrast, the expression of actin, TCR-zeta chain, ZAP-70, and p59(fyn) remained similar in controls and infected mice. Identical results were obtained with sorted B cells and T cells. Surprisingly, a B cell lymphoma line derived from late stage MAIDS mice displayed a normal level of c-Cbl.

Tissue hyperplasia and enhanced T-cell signalling via ZAP-70 in c-Cbl-deficient mice

The c-Cbl protein is tyrosine phosphorylated and forms complexes with a wide range of signalling partners in response to various growth factors. How c-Cbl interacts with proteins, such as Grb2, phosphatidylinositol 3-kinase, and phosphorylated receptors, is well understood, but its role in these complexes is unclear. Recently, the Caenorhabditis elegans Cbl homolog, Sli-1, was shown to act as a negative regulator of epidermal growth factor receptor signalling. This finding forced a reassessment of the role of Cbl proteins and highlighted the desirability of testing genetically whether c-Cbl acts as a negative regulator of mammalian signalling. Here we investigate the role of c-Cbl in development and homeostasis in mice by targeted disruption of the c-Cbl locus. c-Cbl-deficient mice were viable, fertile, and outwardly normal in appearance. Bone development and remodelling also appeared normal in c-Cbl mutants, despite a previously reported requirement for c-Cbl in osteoclast function. However, consistent with a high level of expression of c-Cbl in the hemopoietic compartment, c-Cbl-deficient mice displayed marked changes in their hemopoietic profiles, including altered T-cell receptor expression, lymphoid hyperplasia, and primary splenic extramedullary hemopoiesis. The mammary fat pads of mutant female mice also showed increased ductal density and branching compared to those of their wild-type littermates, indicating an unanticipated role for c-Cbl in regulating mammary growth. Collectively, the hyperplastic histological changes seen in c-Cbl mutant mice are indicative of a normal role for c-Cbl in negatively regulating signalling events that control cell growth. Consistent with this view, we observed greatly increased intracellular protein tyrosine phosphorylation in thymocytes following CD3epsilon cross-linking. In particular, phosphorylation of ZAP-70 kinase in thymocytes was uncoupled from a requirement for CD4-mediated Lck activation. This study provides the first biochemical characterization of any organism that is deficient in a member of this unique protein family. Our findings demonstrate critical roles for c-Cbl in hemopoiesis and in controlling cellular proliferation and signalling by the Syk/ZAP-70 family of protein kinases.

Regulation of beta 1-integrin-mediated cell adhesion by the Cbl adaptor protein

BACKGROUND

Leukocyte activation results in a rapid increase in adhesion to the extracellular matrix due to the activation of beta 1 integrin receptors. A role for phosphatidylinositol (PI) 3-kinase in integrin activation has been proposed, as activation of integrins by many receptors can be blocked by PI 3-kinase inhibitors. One receptor that regulates integrins is the CD28 surface antigen; here, we investigated the mechanisms responsible for CD28-mediated integrin activation.

RESULTS

CD28-mediated integrin activation was blocked by mutation of the binding site for the p85 catalytic subunit of PI 3-kinase in the CD28 cytoplasmic domain, and by expression of a dominant-negative form of the p85 subunit. Substitution of the Src homology 2 (SH2)-binding motif in the CD28 cytoplasmic domain for the corresponding motif in the CD28-related CTLA-4 surface antigen also blocked integrin activation but did not affect the recruitment and activation of PI 3-kinase. Mutations of the CD28 cytoplasmic domain that blocked integrin activation also impaired the tyrosine phosphorylation of the Cbl adaptor protein and the activation of the PI 3-kinase that was associated with Cbl. This Cbl-associated PI 3-kinase was distinct from the PI 3-kinase that coprecipitated with the CD28 cytoplasmic domain. CD28-mediated activation of beta 1 integrins was inhibited by expression of a mutant Cbl protein that shows reduced association with PI 3-kinase.

CONCLUSIONS

Cbl is required for PI-3-kinase-dependent regulation of integrin receptors by CD28. Furthermore, CD28 is coupled to two distinct pools of PI 3-kinase, one directly associated with the CD28 cytoplasmic tail and the other associated with Cbl.

The CBL-related protein CBLB participates in FLT3 and interleukin-7 receptor signal transduction in pro-B cells

The FLT3 receptor tyrosine kinase and its ligand, FL, play an important role in early hematopoietic development. We have found that CBLB, a recently characterized molecule closely related to the CBL protooncogene product, is phosphorylated on tyrosine(s) following FL treatment of JEA2 human pro-B cells and THP1 monocytic cells. Treatment of JEA2 cells with interleukin (IL)-7 induces CBLB phosphorylation as well. FL and IL-7, respectively, induce and increase association of tyrosine-phosphorylated SHC and the p85 subunit of phosphatidylinositol 3'-kinase with CBLB. In these cells, CBLB constitutively binds the GRB2 adaptor predominantly through its N-terminal SH3 domain, to form a complex that is distinct from the GRB2.CBL and GRB2.SOS1 complexes. Together with the fact that CBLB is consistently found in blast cells from acute leukemias and in peripheral blood mononuclear cells, this suggests that CBLB has a role in tyrosine kinase-regulated signaling pathways in many hematolymphoid cells.

Cbl: complex formation and functional implications

Chbl, a 120-kDa proto-oncogene product, whose gene was first identified as part of a transforming gene of a murine retrovirus and whose expression is predominant in haematopoietic cells, consists of an amino-terminal transforming region, a zinc Ring finger, multiple proline-rich stretches, and several potential phosphotyrosine-containing motifs. Cbl is rapidly tyrosine-phosphorylated in response to stimulation of a variety of cell-surface receptors and becomes associated with a number of intracellular signalling molecules such as protein tyrosine kinases, phosphatidylinositol 3-kinase, Crk, and 14-3-3 proteins through different protein-interacting modules, leading to the formation of multimolecular signalling complexes. Cbl and its transforming mutants have been shown to display both negative and positive regulatory activities in protein tyrosine kinase- and Ras-mediated signalling pathways. Nevertheless, the exact biological function of this adaptor protein remains largely unknown. The present review summarizes recent progress in our understanding of the structure, regulation and biological function of Chl and defines open questions for future research.

Ligation of the beta2 integrin triggers activation and degranulation of human eosinophils

Evidence suggests that cellular adhesion is critical for eosinophil effector functions. Here, we tested the hypothesis that an adhesion molecule, specifically beta2 integrin, participates in intracellular signaling events of eosinophils. Eosinophils stimulated with interleukin (IL)-5 and adherent to protein-coated tissue culture plates via beta2 integrin (CD18) showed tyrosine phosphorylation of a number of proteins. Among these proteins, tyrosine phosphorylation of the 105 kD and 115 kD proteins and the product of the c-cbl protooncogene, Cbl, was specifically inhibited using soluble anti-CD18 monoclonal antibody (mAb) to block eosinophil cell adhesion. Furthermore, phosphoinositide turnover of IL-5-stimulated adherent eosinophils was also inhibited by anti-CD18 mAb, suggesting that cellular adhesion plays important roles in eosinophil signal transduction. alphaM beta2 (Mac-1, CD11b/18) was one of the beta2 integrins involved in eosinophil adhesion to protein-coated plates. We found that direct ligation of eosinophil alphaM beta2 with anti-CD11b mAb coupled to polystyrene microbeads induced tyrosine phosphorylation of a 115 kD protein and Cbl. Furthermore, anti-CD11b mAb microbeads induced increases in both phosphoinositide hydrolysis and the eosinophil degranulation response. Control antibodies, such as mouse myeloma IgG1 and anti-HLA class I antigen mAb, did not induce these cellular responses. These results suggest that engagement of beta2 integrin either by cell adhesion or by anti-CD11b mAb triggers activation of an intracellular signaling cascade, including protein tyrosine phosphorylation and phosphoinositide turnover, and subsequent cellular degranulation in human eosinophils. Tyrosine phosphorylation of a 115 kD protein and Cbl may play important roles in adhesion-dependent cellular functions of eosinophils.

Coordinated regulation of the tyrosine phosphorylation of Cbl by Fyn and Syk tyrosine kinases

Cross-linking of the T cell antigen receptor (TCR)-CD3 complex induces rapid tyrosine phosphorylation and activation of Src (Lck and Fyn) and Syk (Syk and Zap-70) family protein tyrosine kinases (PTKs) which, in turn, phosphorylate multiple intracellular substrates. Cbl is a prominent PTK substrate suggesting a pivotal role for it in early signal transduction events. However, the regulation of Cbl function and tyrosine phosphorylation in T cells by upstream PTKs remains poorly understood. In the present study, we used genetic and biochemical approaches to demonstrate that Cbl directly interacts with Syk and Fyn via its N-terminal and C-terminal regions, respectively. Tyr-316 of Syk was required for the interaction with Cbl as well as for the maximal tyrosine phosphorylation of Cbl. However, both wild-type Syk and Y316F-mutated Syk phosphorylated equally well the C-terminal fragment of Cbl in vivo, suggesting the existence of an alternative, N terminus-independent mechanism for the Syk-induced tyrosine phosphorylation of Cbl. This mechanism appears to involve Fyn, since, in addition to its association with the C-terminal region of Cbl, Fyn also associated with Syk and enhanced the Syk-induced tyrosine phosphorylation of Cbl. These findings implicate Fyn as an adaptor protein that facilitates the interaction between Syk and Cbl, and suggest that Src and Syk family PTKs coordinately regulate the tyrosine phosphorylation of Cbl.

Fyn, Yes, and Syk phosphorylation sites in c-Cbl map to the same tyrosine residues that become phosphorylated in activated T cells

Protooncogenic protein c-Cbl undergoes tyrosine phosphorylation in response to stimulation through the receptors for antigens, immunoglobulins, cytokines, and growth factors as well as through the integrins. Tyrosine phosphorylation of c-Cbl may play a functional role in signal transduction, since c-Cbl interacts with many crucial signaling molecules including protein-tyrosine kinases, adaptor proteins, and phosphatidylinositol 3'-kinase. Therefore, it is essential for our understanding of the functions of c-Cbl in signal transduction to identify its tyrosine phosphorylation sites, to determine the protein-tyrosine kinases that phosphorylate these sites, and to elucidate the role of these sites in the interactions of c-Cbl with other signaling proteins. In this report, we demonstrate that tyrosines 700, 731, and 774 are the major tyrosine phosphorylation sites of c-Cbl in T cells in response to pervanadate treatment, as well as in response to TcR/CD3 ligation. Coexpression experiments in COS cells demonstrate that among T cell-expressed Src- and Syk-related protein-tyrosine kinases, Fyn, Yes, and Syk appear to play a major role in phosphorylation of c-Cbl, whereas Lck and Zap phosphorylate c-Cbl ineffectively. Fyn, Yes, and Syk phosphorylate the same sites of c-Cbl that become phosphorylated in stimulated T cells. Among these kinases, Fyn and Yes demonstrate strong binding to c-Cbl, which involves both phosphotyrosine-dependent and phosphotyrosine-independent mechanisms.

CD16-mediated activation of phosphatidylinositol-3 kinase (PI-3K) in human NK cells involves tyrosine phosphorylation of Cbl and its association with Grb2, Shc, pp36 and p85 PI-3K subunit

Phosphatidylinositol 3-kinase (Pl-3K) plays a key role in several cellular processes, including mitogenesis, apoptosis, actin reorganization and vesicular trafficking. The molecular events involved in its activation have not been fully elucidated and several reports indicate that a key event for enzyme activation is the interaction of the SH2 domains of the p85 regulatory subunit of Pl-3K with tyrosine-phosphorylated proteins. In this study, we investigated the involvement of the product of the proto-oncogene c-Cbl in the activation of Pl-3K triggered by CD16 in human NK cells and the possible mechanisms leading to Pl-3K recruitment to the plasma membrane. Our results indicate that stimulation of NK cells through CD16 results in a rapid tyrosine phosphorylation of Cbl, which is constitutively associated with Grb2 and forms an activation-dependent complex with the p85 subunit of Pl-3K. In addition, we detected the presence of the Grb2-associated tyrosine-phosphorylated p36 and Shc proteins in anti-Cbl and anti-p85 immunoprecipitates from CD16-stimulated NK cell lysates. Upon CD16 stimulation, Pl-3K activity was found associated with Cbl and to a lesser extent with Grb2 and Shc as well as with the zeta chain of the CD16 receptor complex. Overall these results suggest that the formation of a complex containing either Shc or pp36 associated with Grb2, Cbl and the p85 subunit of Pl-3K is one of the major mechanisms which might couple CD16 to the Pl-3K pathway in NK cells.

Inhibition of caspase activity prevents anti-IgM induced apoptosis but not ceramide generation in WEHI 231 B cells

In apoptosis induced by Reaper in Drosophila, as well as in a number of other systems, it has been suggested that the increased synthesis of ceramide might be a consequence of the activation of the caspase/ICE (Interleukin-1beta converting enzyme) protease pathway involved in cell death, implying that ceramide generation might often be the result rather than the cause of apoptosis. WEHI 231 B cells have previously been shown to undergo apoptosis following exposure to exogenous ceramide and to produce increased amounts of ceramide in response to anti-IgM crosslinking. We show here that in WEHI 231 cells a peptide inhibitor of caspase activity blocks cell death in response to both anti-IgM and exogenous ceramide. However, the induction of ceramide synthesis by WEHI 231 cells in response to anti-IgM crosslinking is not blocked by this peptide. These results indicate that antigen receptor induced ceramide generation in WEHI 231 cells does not require caspase activation, and support the view that ceramide generation in immature B cells may be the cause rather than the consequence of activation of the caspase dependent death pathway.

Costimulation through CD28 suppresses T cell receptor-dependent activation of the Ras-like small GTPase Rap1 in human T lymphocytes

Members of the Ras superfamily of small molecular weight GTPases play diverse and critical roles in mediating cellular responses to extracellular stimuli, including mitogenesis, cytoskeletal maintenance and rearrangement, and integrin activation. In T lymphocytes, biochemical and genetic evidence demonstrate that Ras plays an essential role in coupling T cell receptor ligation to signaling cascades required for T cell proliferation and development. Recent observations that C3G, a guanine nucleotide exchange factor specific for the Ras-related GTPase Rap1, is recruited into tyrosine-phosphorylated protein signaling complexes in activated T cells have suggested that Rap1 may also play a role in T cell activation. Utilizing a recently developed technique for detection of endogenous, GTP-bound Rap1, we have found that Rap1, but not Rap2, is transiently activated following T cell receptor stimulation of normal human T lymphocytes. Increases in intracellular calcium is both necessary and sufficient to induce Rap1 activation. Remarkably, costimulation of T cells with mitogenic anti-CD28 antibody completely abolished T cell receptor-dependent activation of Rap1. This report demonstrates a potential role for Rap1 in T cell receptor signaling and suggests inactivation of Rap1 as a candidate target of CD28-dependent costimulatory signals required for T cell antigen responsiveness.

A novel, multifuntional c-Cbl binding protein in insulin receptor signaling in 3T3-L1 adipocytes

The protein product of the c-Cbl proto-oncogene is prominently tyrosine phosphorylated in response to insulin in 3T3-L1 adipocytes and not in 3T3-L1 fibroblasts. After insulin-dependent tyrosine phosphorylation, c-Cbl specifically associates with endogenous c-Crk and Fyn. These results suggest a role for tyrosine-phosphorylated c-Cbl in 3T3-L1 adipocyte activation by insulin. A yeast two-hybrid cDNA library prepared from fully differentiated 3T3-L1 adipocytes was screened with full-length c-Cbl as the target protein in an attempt to identify adipose-specific signaling proteins that interact with c-Cbl and potentially are involved in its tyrosine phosphorylation in 3T3-L1 adipocytes. Here we describe the isolation and the characterization of a novel protein that we termed CAP for c-Cbl-associated protein. CAP contains a unique structure with three adjacent Src homology 3 (SH3) domains in the C terminus and a region showing significant sequence similarity with the peptide hormone sorbin. Both CAP mRNA and proteins are expressed predominately in 3T3-L1 adipocytes and not in 3T3-L1 fibroblasts. CAP associates with c-Cbl in 3T3-L1 adipocytes independently of insulin stimulation in vivo and in vitro in an SH3-domain-mediated manner. Furthermore, we detected the association of CAP with the insulin receptor. Insulin stimulation resulted in the dissociation of CAP from the insulin receptor. Taken together, these data suggest that CAP represents a novel c-Cbl binding protein in 3T3-L1 adipocytes likely to participate in insulin signaling.

Interleukin-3 induces association of the protein-tyrosine phosphatase SHP2 and phosphatidylinositol 3-kinase with a 100-kDa tyrosine-phosphorylated protein in hemopoietic cells

We have observed previously the co-immunoprecipitation of the p85 subunit of phosphatidylinositol-3 kinase (PI3K) and SHP2 in murine lymphohemopoietic cells after stimulation with interleukin-3. We have investigated this interaction in more detail and now report the identification of a potentially novel 100-kDa protein (termed p100), which is inducibly phosphorylated on tyrosine after interleukin-3 treatment and which co-immunoprecipitates with both p85 PI3K and SHP2. The Src homology region 2 domains of both p85 and SHP2 appear to mediate their interactions with p100. Sequential precipitation analyses suggest that these interactions are direct and do not involve Grb2, and that the same p100 protein, or a portion of it, interacts with both p85 and SHP2, implying that p100 may serve to link these two proteins. Far Western blotting with both full-length p85 and isolated p85 Src homology region 2 domains supports this view. Interestingly, p100 also appears to be a substrate for the SHP2 phosphatase activity. In addition, p100 is precipitated by Grb2-glutathione S-transferase fusion proteins, an interaction largely mediated by the Grb2 SH3 domains. p100 appears to be distinct from JAK2, Vav, STAT5, and c-Cbl. Although largely cytosolic, p100 can be detected associated with SHP2 and PI3K in crude membrane fractions after interleukin-3 stimulation. We propose that p100 plays a role as an adaptor molecule, linking PI3K and SHP2 in IL-3 signaling.

Phosphotyrosine binding domain-dependent upregulation of the platelet-derived growth factor receptor alpha signaling cascade by transforming mutants of Cbl: implications for Cbl's function and oncogenicity

Recent studies have demonstrated that Cbl, the 120-kDa protein product of the c-cbl proto-oncogene, serves as a substrate of a number of receptor-coupled tyrosine kinases and forms complexes with SH3 and SH2 domain-containing proteins, pointing to its role in signal transduction. Based on genetic evidence that the Caenorhabditis elegans Cbl homolog, SLI-1, functions as a negative regulator of the LET-23 receptor tyrosine kinase and our demonstration that Cbl's evolutionarily conserved N-terminal transforming region (Cbl-N; residues 1 to 357) harbors a phosphotyrosine binding (PTB) domain that binds to activated ZAP-70 tyrosine kinase, we examined the possibility that oncogenic Cbl mutants may activate mitogenic signaling by deregulating cellular tyrosine kinase machinery. Here, we show that expression of Cbl-N and two other transforming Cbl mutants (CblY368 delta and Cbl366-382 delta or Cb170Z), but not wild-type Cbl, in NIH 3T3 fibroblasts leads to enhancement of endogenous tyrosine kinase signaling. We identified platelet-derived growth factor receptor alpha (PDGFR alpha) as one target of mutant Cbl-induced deregulation. In mutant Cbl transfectants, PDGFR alpha was hyperphosphorylated and constitutively complexed with a number of SH2 domain-containing proteins. PDGFR alpha hyperphosphorylation and enhanced proliferation of mutant Cbl-transfected NIH 3T3 cells were drastically reduced upon serum starvation, and PDGF-AA substituted for the maintenance of these traits. PDGF-AA stimulation of serum-starved Cbl transfectants induced the in vivo association of transfected Cbl proteins with PDGFR alpha. In vitro, Cbl-N directly bound to PDGFR alpha derived from PDGF-AA-stimulated cells but not to that from unstimulated cells, and this binding was abrogated by a point mutation (G306E) corresponding to a loss-of-function mutation in SLI-1. The Cbl-N/G306E mutant protein, which failed to induce enhanced growth and transformation of NIH 3T3 cells, also failed to induce hyperphosphorylation of PDGFR alpha. Altogether, these findings identify a novel mechanism of Cbl's physiological function and oncogenesis, involving its PTB domain-dependent direct interaction with cellular tyrosine kinases.

The complexity of signaling pathways activated by the BCR

Cross-linking of the B cell antigen receptor (BCR) leads to the activation of three types of intracellular protein tyrosine kinases. These tyrosine kinases then phosphorylate signaling components to activate a variety of signaling reactions, including phosphatidylinositol 4,5-bisphosphate hydrolysis, Ras activation, and phosphatidylinositol 3-kinase activation. Each of these signaling reactions, and also the signaling molecules Vav and HS1, appears to be important for at least some of the many types of B cell responses to antigen. The complexity of BCR signaling reactions may be required to allow the B cell to respond in a number of distinct ways to antigen (proliferation, survival, apoptosis, maturational arrest, etc.) depending on the maturation state of the B cell, the location in the body, the physical nature of the antigen, and the possible presence of the antigen in complex with antibody or complement components.

Erythropoietin and Interleukin-3 Activate Tyrosine Phosphorylation of CBL and Association With CRK Adaptor Proteins

Transformation of hematopoietic cells by the Bcr-abl oncoprotein leads to constitutive tyrosine phosphorylation of a number of cellular polypeptides that function in normal growth factor-dependent cell proliferation. Recent studies have shown that the CrkL adaptor protein and the Cbl protooncoprotein are constitutively tyrosine phosphorylated and form a preformed complex in cells expressing Bcr-abl. In the current study, we have examined cytokine-dependent tyrosine phosphorylation of Cbl and its association with Crk proteins. Erythropoietin (EPO) and interleukin-3 induced a dose and time-dependent tyrosine phosphorylation of Cbl in both EPO-dependent Ba/F3 and DA-3 transfectants, and the erythroid cell line HCD-57. Furthermore, once phosphorylated, Cbl associated with Crk adaptor proteins. Of the three Crk isoforms expressed in hematopoietic cells (CrkL, CrkII, and CrkI), tyrosine phosphorylated Cbl binds preferentially to CrkL and CrkII. The amount of Cbl associated with CrkL and CrkII exceeded the fraction of Cbl associated with Grb2 indicating that unlike other receptor systems, the Cbl-Crk association represents the dominant complex of Cbl in growth factor-stimulated hematopoietic cells. In factor-dependent hematopoietic cell lines, CrkL constitutively associated with the guanine nucleotide release factor, C3G, which is known to interact via Crk src-homology 3 (SH3) domains. Our data suggest that the inducible Cbl-Crk association is a proximal component of a signaling pathway downstream of multiple cytokine receptors.

Purification and Molecular Cloning of SH2- and SH3-Containing Inositol Polyphosphate-5-Phosphatase, Which Is Involved in the Signaling Pathway of Granulocyte-Macrophage Colony-Stimulating Factor, Erythropoietin, and Bcr-Abl

Grb2/Ash and Shc are the adapter proteins that link tyrosine-kinase receptors to Ras and make tyrosine-kinase functionally associated with receptors and Ras in fibroblasts and hematopoietic cells. Grb2/Ash and Shc have the SH3, SH2, or phosphotyrosine binding domains. These domains bind to proteins containing proline-rich regions or tyrosine-phosphorylated proteins and contribute to the association of Grb2/Ash and Shc with other signaling molecules. However, there could remain unidentified signaling molecules that physically and functionally interact with these adapter proteins and have biologically important roles in the signaling pathways. By using the GST fusion protein including the full length of Grb2/Ash, we have found that c-Cbl and an unidentified 135-kD protein (pp135) are associated with Grb2/Ash. We have also found that they become tyrosine-phosphorylated by treatment of a human leukemia cell line, UT-7, with granulocyte-macrophage colony-stimulating factor (GM-CSF ). We have purified the pp135 by using GST-Grb2/Ash affinity column and have isolated the full-length complementary DNA (cDNA) encoding the pp135 using a cDNA probe, which was obtained by the degenerate polymerase chain reaction based on a peptide sequence of the purified pp135. The cloned cDNA has 3,958 nucleotides that contain a single long open reading frame of 3,567 nucleotides, encoding a 1,189 amino acid protein with a predicted molecular weight of approximately 133 kD. The deduced amino acid sequence reveals that pp135 is a protein that has one SH2, one SH3, and one proline-rich domain. The pp135, which contains two motifs conserved among the inositol polyphosphate-5-phosphatase proteins, was shown to have the inositol polyphosphate-5-phosphatase activity. The pp135 was revealed to associate constitutively with Grb2/Ash and inducibly with Shc using UT-7 cells stimulated with GM-CSF. In the cell lines derived from human chronic myelogenous leukemia, pp135 was constitutively tyrosine-phosphorylated and associated with Shc and Bcr-Abl. These facts suggest that pp135 is a signaling molecule that has a unique enzymatic activity and should play an important role in the signaling pathway triggered by GM-CSF and in the transformation of hematopoietic cells caused by Bcr-Abl.

Serine phosphorylation of Cbl induced by phorbol ester enhances its association with 14-3-3 proteins in T cells via a novel serine-rich 14-3-3-binding motif

Stimulation of the T cell antigen receptor (TCR).CD3 complex induces rapid tyrosine phosphorylation of Cbl, a protooncogene product which has been implicated in intracellular signaling pathways via its interaction with several signaling molecules. We found recently that Cbl associates directly with a member of the 14-3-3 protein family (14-3-3tau) in T cells and that the association is increased as a consequence of anti-CD3-mediated T cell activation. We report here that phorbol 12-myristate 13-acetate stimulation of T cells also enhanced the interaction between Cbl and two 14-3-3 isoforms (tau and zeta). Tyrosine phosphorylation of Cbl was not sufficient or required for this increased interaction. Thus, cotransfection of COS cells with Cbl plus Lck and/or Syk family protein-tyrosine kinases caused a marked increase in the phosphotyrosine content of Cbl without a concomitant enhancement of its association with 14-3-3. Phorbol 12-myristate 13-acetate stimulation induced serine phosphorylation of Cbl, and dephosphorylation of immunoprecipitated Cbl by a Ser/Thr phosphatase disrupted its interaction with 14-3-3. By using successive carboxyl-terminal deletion mutants of Cbl, the 14-3-3-binding domain was mapped to a serine-rich 30-amino acid region (residues 615-644) of Cbl. Mutation of serine residues in this region further defined a binding motif distinct from the consensus sequence RSXSXP, which was recently identified as a 14-3-3-binding motif. These results suggest that TCR stimulation induces both tyrosine and serine phosphorylation of Cbl. These phosphorylation events allow Cbl to recruit distinct signaling elements that participate in TCR-mediated signal transduction pathways.

Thrombopoietin induces activation of the phosphatidylinositol-3' kinase pathway and formation of a complex containing p85PI3K and the protooncoprotein p120CBL

Thrombopoietin (TPO) promotes megakaryocyte growth and development. Its receptor, c-MPL, is restricted to cells of megakaryocytic lineage and stem cells. We have previously shown that activation of c-MPL by thrombopoietin rapidly activates at least two cytoplasmic tyrosine kinases, JAK2 and TYK2, after ligand binding. Phosphatidylinositol-3' kinase (PI3K) has been shown to play an important role in downstream signaling for many receptors. Thrombopoietin was found to also rapidly activate phosphatidylinositol-3' kinase, and the phosphatidylinositol-3' kinase inhibitor wortmannin decreased proliferation of thrombopoietin-stimulated cells, implying that phosphatidylinositol-3' kinase may have a regulatory role in thrombopoietin signaling. In immunoprecipitation studies, the regulatory subunit of phosphatidylinositol-3' kinase, p85PI3K, associated with several tyrosine phosphoproteins, and the major phosphoprotein was a 120 kDa protein identified as p120CBL. The phosphatidylinositol-3' kinase-enzyme activity in p120CBL immunoprecipitates was elevated in thrombopoietin-stimulated cells as compared to immunoprecipitates from unstimulated cells. p120CBL may be involved in signaling pathways activated by c-MPL which involve phosphatidylinositol-3' kinase.

The proto-oncogene product p120(cbl) links c-Src and phosphatidylinositol 3'-kinase to the integrin signaling pathway

Integrin-mediated cell adhesion triggers intracellular signaling cascades, including tyrosine phosphorylation of intracellular proteins. We show in this report that p120(cbl) (Cbl), the 120-kDa c-cbl proto-oncogene product, becomes tyrosine-phosphorylated during integrin-mediated macrophage cell adhesion to extracellular matrix substrata and anti-integrin antibodies. This tyrosine phosphorylation does not occur when cells attach to polylysine, to which cells adhere in a nonspecific fashion. It also does not take place when adhesion-induced reorganization of the cytoskeleton is inhibited with cytochalasin D. In contrast to the rapid and transient tyrosine phosphorylation of Cbl by CSF-1 stimulation, tyrosine phosphorylation of Cbl by cell attachment was gradual and persistent. Tyrosine-phosphorylated Cbl was found to form complexes with the SH2 domain-containing signaling proteins Src and phosphatidylinositol 3-kinase; in vitro kinase assays demonstrated that these kinases were active in the Cbl complexes following integrin ligand binding. Furthermore, Cbl was found to translocate to the plasma membrane in response to cell adhesion to fibronectin. These observations suggest that Cbl serves as a docking protein and may transduce signals to downstream signaling pathways following integrin-mediated cell adhesion in macrophages.

Ras-dependent, Ca2+-stimulated activation of nuclear factor of activated T cells by a constitutively active Cbl mutant in T cells

T cell receptor (TCR) stimulation induces rapid tyrosine phosphorylation of cellular proteins, including Cbl, a protooncogene product whose function remains unclear. As a first step toward elucidating the function of Cbl in TCR-initiated signaling, we evaluated the ability of wild-type Cbl or a transforming Cbl mutant (70Z/3) to induce transcriptional activation of a nuclear factor of activated T cells (NFAT) element derived from the interleukin 2 (IL2) promoter in transiently cotransfected Jurkat-TAg T cells. 70Z/3, but not Cbl, caused NFAT activation which was significantly enhanced by stimulation with calcium ionophore, and was drastically reduced by cyclosporin A pretreatment. A point mutation of a potential phosphatidylinositol 3-kinase (PI3-K) binding site (Y731EAM to Y731EAC) in 70Z/3 disrupted the association of PI3-K with 70Z/3, but did not reduce the induction of NFAT activity, suggesting that the interaction between Cbl and PI3-K is not required in the 70Z/3-mediated induction of NFAT. Additional mapping studies indicated that defined deletions of C-terminal 70Z/3 sequences affected to a variable degree its ability to stimulate NFAT activity. Strikingly, deletion of 346 C-terminal residues augmented this activity, whereas removal of 20 additional residues abolished it. Coexpression of dominant negative Ras abrogated the basal or ionomycin-stimulated, 70Z/3-mediated NFAT activation, suggesting a functional Ras is required for this activation. These results implicate Cbl in Ras-dependent signaling pathways which lead to NFAT activation.